1. Field of the Invention
The invention is a blood reservoir with at least one pliable wall. It innovates the art in at least two ways. First, the compliant reservoir is sealed within a rigid housing allowing control of the xe2x80x9catmosphericxe2x80x9d pressure surrounding the bag, and therefore the pressure at which the bag would collapse. This first invention enables vacuum augmented venous drainage (VAVD) with a collapsible soft-shell reservoir (i.e. venous bag) and is particularly useful for cardiopulmonary bypass. Second, the invention has a tubular structure with a screened wall that improves gas bubble removal from blood transiting the collapsible reservoir.
2. Description of the Prior Art
Blood is routinely pumped outside the body during dialysis, cardiopulmonary bypass, and long term cardiac and/or respiratory support (e.g. extracorporeal membrane oxygenation, ECMO). In general, blood flows from the venous side of the patient to a venous reservoir that is usually maintained at atmospheric pressure. Blood flow from the patient to the reservoir is a function of the resistance of the fluid conduit between patient and reservoir, and the pressure difference between patient and reservoir. When the reservoir is maintained at atmospheric pressure, that pressure difference is the height difference between patient and reservoir; the resulting flow is referred to as gravity drainage. Venous drainage by gravity alone provides inadequate return during procedures such as minimally invasive cardiac surgery and bypass via femoral cannulation. Usually it is the resistance of the venous cannula that limits the flow achievable. Vacuum augmented venous drainage (VAVD) is a technique that overcomes flow limitations by applying suction to the hard shell reservoir thereby increasing the pressure difference between the venous cannulation site and venous reservoir. VAVD allows for a decrease in the inner diameter of the venous line, thereby reducing prime volume, and enabling the use of a smaller cannula, which translates to an easier insertion, a better surgical view and a smaller surgical incision. This method precludes the use of the safer soft-shell closed venous reservoir (venous bag) unless a more expensive and complicated two-pump system is used (see McKusker K, Hoffman D, Maldarelli W, Toplitz S, and Sisto D. High-flow femoro-femoral bypass utilizing small cannulae and a centrifugal pump on the venous side. Perfusion 1992; 7:295-300).
Clinically, a venous bag is used because it provides significant safety features. If the bag empties, it collapses, thereby preventing gross air from being pumped to the patient. It usually has no air-blood interface, and it requires no antifoam agents that can embolize into the blood. A recent study by Schonberger et al (Schonberger JPAM, Everts PAM, and Hoffmann JJ. xe2x80x9cSystemic blood activation with open and closed venous reservoirs. Annals of Thoracic Surgery, 1995; Vol. 59, pages 1549-55) comparing the hard shell to the bag reservoir found significantly lower blood activation, shed blood loss, crystalloid infusion, and hemolysis, and less donor blood infusion with the bag reservoir. Schonberger""s group recommended against routine use of an open (hard shell) venous reservoir system. Currently, a slight negative pressure applied to the venous line (to facilitate blood drainage) using a single pump can only be achieved with less desirable hard shell venous reservoirs. It is impossible to apply negative pressure to current soft-shell reservoirs, but it is possible with one preferred embodiment of the present invention.
In an open, hard shell reservoir, air escapes by floating to the top of the reservoir. In a bag reservoir, air floats to the top but must be actively eliminated. This can be done manually with a syringe, or more frequently with a roller pump operating slowly so as to continuously pump fluid to the cardiotomy reservoir. With either method, a sudden large volume of air can overwhelm the air removal system and cause disastrous consequences, especially without a vigilant perfusionist. With one preferred embodiment of the present invention, air would be eliminated automatically without a roller pump or intervention by the perfusionist, and priming of the extracorporeal circuit would be facilitated through faster air removal utilizing either a floating ball valve or a hydrophobic membrane. Currently there are devices used in the CPB circuit that incorporate hydrophobic membranes that remove air yet do not allow blood to cross (e.g. Model # AutoVent-SV, Pall Corp Glen Cove N.Y.). Studies with filters used in these applications have shown that the membranes clear air from water almost indefinitely (many days), even if high suction is applied, without reducing gas transfer rate over time. However, when the membrane is exposed to blood, especially when high suction is applied, a film forms on the membrane over time, causing a significant increase in resistance to gas flow. The present invention incorporates designs and means to reduce this problem and extend the life of the membrane when used with blood. Likewise, U.S. Pat. No. 3,849,071 shows a floating ball within a blood filter that supposed to open a purge port when air enters and close when the blood level rises. However, as described, it is a physical impossibility for the ball to xe2x80x9cfallxe2x80x9d and open the purge port because, as shown, the weight of the floating ball is insufficient to overcome the force holding the ball against the purge port. With the present invention, the relative weight of the ball, the internal diameter of the purge port, and the suction applied to the purge port are designed to assure that the ball will drop to open the purge port in response to air level in the venous reservoir.
With prior art soft-shell reservoirs, air may be trapped at the top of the liquid by the collapsed walls of the reservoir, see FIG. 1a. U.S. Pat. No. 4,622,032 illustrates a soft shell reservoir having an inlet tube extending from the bottom half way into the reservoir. Reference 13, sold by Johnson and Johnson (and later by Medtronic) shows a soft shell reservoir with an inline tube extending, along one side of the bag, to the gas purge port at a 45xc2x0 incline. This design has a rigid fluid path between blood inlet and gas purge port. However, this design is not as conducive to air removal as a vertical fluid path would be. In addition, the tube extending between inlet tube and purge port had an ID of ⅝xe2x80x3, or only 25% greater diameter than the inlet tube. Thus, the velocity of the liquid in the column slows to only 64% of the inlet velocity. In another aspect of the present invention, a vertical path is provided from the blood inlet at the bottom of the bag to the gas purge port at the top of the bag, such path limiting the aforementioned problem of trapped air. The vertical path also has a large enough diameter that slows the velocity of the liquid to at least 25%, and more preferably to at least 15% of the inlet velocity. As will be shown, lower blood velocity is more conducive to bubble removal.
State of the art soft shell venous reservoirs with a screen are designed such that a large portion of the screen contacts the internal walls of the bag, thereby increasing the resistance to blood flow across the screen, and rendering that portion of the screen ineffective, at least partially. This contact between the screen and the walls of the bag increases as the volume in the reservoir decreases. The present invention reduces that problem by preventing the external walls of the venous reservoir from contacting the screen.
The indication of blood level in present soft shell venous reservoir is very inaccurate and low level, or air-in-the-reservoir, alarms are not reliable because many are designed for hard shell reservoirs. The present invention provides effective means to alarm at low blood levels and in the soft shell venous reservoir.
Currently, at the end of the bypass procedure, the patient is weaned off the heart lung machine by reducing the bypass flow. This is achieved by partially clamping the venous line and decreasing the speed of the arterial pump. Once off bypass, the blood left in the venous reservoir is gradually pumped back to the venous side of the patient. Another aspect of the invention allows the user to adjust the positive pressure applied to the blood within the venous reservoir. By being able to increase the pressure of the venous reservoir, the user can effectively reduce venous drainage or perfuse the blood back to the patient. This is not possible with current venous reservoir bags and may be dangerous with hard shell reservoirs (i.e., air may be pushed to the patient).
The inventor has also previously described an inline bladder (The Better-Bladder(trademark), see U.S. Pat. No. 6,039,078), a device with a thin walled, sausage shaped bladder sealed inside a clear, rigid housing. Since the bladder is made from a single piece of tubing, the blood path is smooth with no flow discontinuities. The bladder portion is sealed within the housing that has an access port to the housing space outside the bladder. Because of its thin wall, the enlarged section can easily collapse. Thus, it can serve as an inline reservoir, providing compliance in the venous line to reduce the pressure pulsations at the pump inlet. The Better-Bladder also transmits the blood pressure flowing through it across its thin wall, allowing pump inlet pressure to be measured noninvasively by measuring the gas pressure of the housing via the gas port. The degree of xe2x80x9cgravity drainagexe2x80x9d is user-adjustable by setting the negative pressure in the Better-Bladder housing. If the suction generated by the venous pump becomes too great, the pump is slowed or stopped by a pump controller. The Better-Bladder does not have a gas purge port or a screen to inhibit gas bubbles. It is also much smaller, having nominal volume of 80 ml for adult perfusion as compared to over 1,000 ml for a venous reservoir.
The present invention incorporates improved designs for venous reservoirs that provide the benefits of prior art devices (limiting pump suction using the safer, pliable blood reservoir) while avoiding their disadvantages (air entrapment, inability to utilize VAVD, required vigilance for air removal). Further, its advantages and uniqueness are also enhanced by providing the user with means to adjust the degree of suction applied for venous augmentation and assuring that a greater area of the screen is effective in liquid transport.
Briefly, the present invention in its simplest form consists of a blood reservoir having at least one pliable wall, a blood inlet, a blood outlet, and a gas purge port. In one preferred embodiment, a first structure having tubular cross section and semirigid wall is placed above said inlet thereby providing a first path for undesirable bubbles entering the reservoir to move to the top where they are eliminated via said gas purge port. The first structure preferably has an effective cross section that is larger than the ID of said inlet tube thereby slowing any blood flow and allowing more favorable conditions (longer time, lower drag) for gas bubbles to float upward. The wall of the first structure is sufficiently rigid to prevent collapse of said pliable wall from blocking said first path. In another preferred embodiment, the pliable wall of the venous reservoir is sealed externally, forming a pressure chamber external to the venous reservoir. Controlled suction applied to said external chamber is transmitted across said pliable wall thereby controlling the negative pressure of the blood at which said pliable wall moves.
It is therefore the objective of the present invention to provide an improved venous blood reservoir with at least one pliable wall that provides a path for gas bubbles entering the inlet to move unhindered up to the gas purge port.
A further objective of the present invention is an improved venous blood reservoir, having at least one pliable wall, allowing the user to adjust the negative pressure applied to said pliable wall thereby allowing for augmented venous drainage.
A further objective of the present invention is an improved venous blood reservoir designed to maintain its external wall from contacting the screen material and thereby reducing the resistance to blood flow across the screen.
Yet another objective of the present invention is to incorporate a one way valve at the outlet of the venous reservoir, said valve preventing blood from being sucked into the venous reservoir when pressure at the outlet of the venous reservoir is positive relative to the liquid pressure in the venous reservoir.
Another objective of the present invention is to provide an improved venous blood reservoir with at least one pliable wall that when placed at the pump inlet, provides compliance that reduces pressure fluctuations at said pump inlet.
Another objective of the present invention is to provide an improved venous blood reservoir with at least one pliable wall that when placed at the pump inlet, provides automated means to eliminate air.
Another objective of the present invention is to provide an improved venous blood reservoir with at least one pliable wall and with a relatively large gas purge port, said port providing a more volumetrically effective gas purge, and one that is less traumatic to the blood.
Another objective of the present invention is to provide automated means to detect air in the venous reservoir and utilize said means to alarm the user or control the suction used to remove air from the venous reservoir.
Other objectives, features and advantages of the present invention will become apparent by reference to the following detailed description of the presently preferred, but nonetheless illustrative, embodiments thereof with reference to the accompanying drawings therein.